Added tox.ini and improved travis.yml

[ZuluPro]

I added a tox file to ease test across Python & Django versions.
I also improved the travis file to use the tox configuration.

[coveralls]

Coverage remained the same at 92.718% when pulling cb9eed6 on cloudspectatordevelopment:tox into b0cba39 on graphql-python:master.

[ZuluPro]

@sciyoshi Any news ?

[ZuluPro]

@sciyoshi Any news ?

[sciyoshi]

Looks fine to me, but I don't have commit access. Please ask @syrusakbary or @jm2242 to review + merge.

[jm2242]

I'm happy to take a look but I'm not sure I'm involved in the project enough to have a say!

[firaskafri]

@ZuluPro Interested to update this pull request?

[zbyte64]

The tests seem to only fail for Django < 1.10 ; which we don't currently test. We should go ahead and disable those environments.

[ZuluPro]

Hello @firaskafri @zbyte64

I updated my PR, removed <Django 1.10

[zbyte64]

Thank you for this PR

[phalt]

I'd like to tidy up some of the PRs, so if we still want to work on this let me know or I will close this in 1 week.

[dulmandakh]

LGTM. @ZuluPro could you please remove 1.10 and 2.0 as these are not supported anymore. Also resolve the conflict.

[phalt]

Please resolve conflicts

[ZuluPro]

@phalt @zbyte64 : Rebased !

Note:
Django master's tests are failing due to a six import:

File "/home/travis/build/graphql-python/graphene-django/graphene_django/settings.py", line 18, in <module>
    from django.utils import six
ImportError: cannot import name 'six' from 'django.utils' (/home/travis/build/graphql-python/graphene-django/.tox/py3.7-djangomaster/lib/python3.7/site-packages/django/utils/__init__.py)```

[dulmandakh]

@ZuluPro @phalt I think we need to add six as dependency and import it directly, rather than from Django.

[dulmandakh]

@phalt let's merge this, and fix django master failure in another PR.